AMD Bulldozer topology regression since 4.6

Brice Goglin
2016-11-29T07:03:29Z

Hello

Since Linux 4.6 (and still in 4.9-rc5 at least), both AMD Bulldozer
cores of a single dual-core compute unit report the same core_id:

$ cat /sys/devices/system/cpu/cpu{?,??}/topology/core_id
0
0
1
1
2
2
3
0
3
[...]

Before 4.5 (and for a very long time), the kernel reported different
core_ids:

0
1
2
3
4
5
6
7
0
[...]

This causes user-space tools (at least all that rely on hwloc for
topology discovery) to think the processor has dual-threaded cores
instead of dual-core compute-unit modules.

The cause is likely this patch, which seems to assume both cores of a
same compute-unit have the same ID, which is wrong?

commit 8196dab4fc159943df6baaac04973bb1accb7100
Author: Borislav Petkov <bp@suse.de>
Date: Fri Mar 25 15:52:36 2016 +0100

x86/cpu: Get rid of compute_unit_id

It is cpu_core_id anyway.

Signed-off-by: Borislav Petkov <bp@suse.de>
Link: http://lkml.kernel.org/r/1458917557-8757-3-git-send-email-bp@alien8.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

Thanks
Brice